This invention relates to an optical disk drive, and more particularly, to an improvement in a servo system using a focus error signal.
In an optical disk drive for recording information on an optical disk such as a compact disc (CD), a digital versatile disc (DVD), or a Blu-ray disc (BD), the information is reproduced or recorded by applying the laser light to an information recording surface of the optical disk and detecting reflected light of the laser light while rotating the optical disk at high speed.
In the optical disk drive, an optical pickup that applies laser light and includes a photodetector for detecting the reflected light is driven in a direction opposed to a surface of the optical disk (focus direction) and in a radial direction of the optical disk (tracking direction (or sliding direction)) to thereby perform servo control so that the laser light is brought into focus on the information recording surface of the optical disk. It is widely known that a focus error signal and a tracking error signal from the laser light detected by the photodetector are used for the servo control in the focus direction and the servo control in the tracking direction, respectively.
The servo control as described above is widely known as having a problem that the tracking error (hereinafter, referred to as “TE”) signal may leak into the focus error (hereinafter, referred to as “FE”) signal when the optical pickup is caused to move in the tracking direction. The crosstalk raises a problem that an unnecessary current may be supplied to an actuator for driving the optical pickup and that a focus servo of the optical pickup may be saturated to be brought out of focus while moving in the tracking direction.
The following technologies are known as technologies for suppressing a phenomenon in which the above-mentioned TE signal leaks into the FE signal.
JP 4-98625 A is known as a technology for suppressing an influence of the TE signal that leaks into the FE signal in a focus servo system. In JP 4-98625 A, in a compensator of a focus servo controller, a limiter is placed between an emphasis at a preceding stage and a de-emphasis at a subsequent stage, and a drive signal of the focus servo is output from the emphasis at the subsequent stage. An object of JP 4-98625 A is to suppress a specific frequency band into which the TE signal leaks by increasing a gain of the specific frequency band by the emphasis at the preceding stage, attenuating an output from the emphasis by the limiter, and reducing the gain of the specific frequency band by the de-emphasis at the subsequent stage.
Further, JP 2006-216132 A is known as a technology for suppressing an out-of-focus state by providing crosstalk level detection means for the FE signal and lowering a focus gain if crosstalk is equal to or higher than a threshold value.
Further, JP 2001-84605 A is known as a technology in which a crosstalk amount of the TE signal into the FE signal is measured by groove crossing amount detection means for detecting an influence exerted when a laser light beam crosses a groove of the optical disk and if the crosstalk amount is large, a gain and a disk rotational speed of a focus servo system are adjusted.
Further, each of JP 3480337 B and JP 3813826 B is known as a technology for canceling FE signal crosstalk with respect to the FE signal by using a compensation signal extracted from the TE signal.